The occurrence of Alexandrium species involved in paralytic shellfish poisoning (PSP) in Thau lagoon (French Mediterranean) (Fig. 1) is known since 1998 [1-2]. Blooms usually occur in spring and autumn, and cause economic damage to shellfish farming that represents 10% of national production. Maximal vegetative cell densities reached 4.5 × 106 cell L-1 and 16 × 106 cell L-1 in October 2003 and November 2004 respectively. Such cell concentrations resulted in red tides during these periods (Fig. 2). Regarding species identification, thecal plate observations and the detection of complex toxin profiles in natural populations sampled in 1998 originally suggested the co-existence of Alexandrium tamarense and Alexandrium catenella